This invention relates to a method and apparatus for driving a brushless motor and, more particularly, to such a technique wherein an n-phase m-pole (wherein n and m are integers) brushless motor is driven without use of physical rotary position detectors, such as Hall elements, photosensors, or the like.
Brushless DC motors have found wide applications and general use in different industries. Typically, a rotor is formed of one or more pairs of magnetic pole pieces, such as permanent magnets, which rotate relative to a plural-phase stator. In many brushless DC motor constructions, the stator is comprised of three phases of stator windings, each phase being energized individually or in combination and in a predetermined sequence such that the magnetic forces induced between the stator windings and the magnetic rotor rotatably drive the rotor.
In one construction, drive current flows through each phase in only a single direction, thus giving rise to the designation "unipolar motor". In another arrangement drive current flows through a given phase in two different directions (at two different times), thus giving rise to the designation "bipolar motor". In both unipolar and bipolar motors, the phases are energized in sequence so as to produce a rotating magnetic force to drive the magnetic rotor.
Generated motor torque from phase to phase is generally sinusoidal, and optimal motor operation obtains if a respective phase is energized to generate a positive torque when the rotor rotates to a particular position relative to that phase. This particular position is a function of the mechanical construction of the stator.
Since the time at which a phase is energized is closely correlated to the angular position of the rotor, proper control over the brushless DC motor generally requires a sensing of the angular position of the rotor (sometimes referred to simply as "sensing the motor position"). Heretofore, rotor position has been sensed by the use of physical position detectors, such as Hall effect devices, optical sensors, and the like. Rotor constructions with integrated Hall elements are known, and these Hall elements generally are formed on the stator structure to sense the magnetic poles of the rotor as the rotor moves therepast. Thus, the Hall elements generate signals which represent the motor position. Usually, pulses are derived from the so-called Hall signals; and these Hall pulses are used to trigger drive signals which, in turn, energize the respective stator phases.
In a similar manner, optical sensors have been used to sense indicia mounted on or rotatable with the rotor, thereby sensing the rotary position of the motor. Sensor-generated signals, typically pulses, represent when the rotor has arrived at a predetermined location, whereupon the stator windings may be energized to create the proper rotary torque.
As motor driven devices have been reduced in size, the drive motors themselves have been miniaturized. Also, economic pressures have dictated lower costs for brushless DC motors. The combination of miniaturization and cost reduction has encouraged a solution to the problem of driving a brushless DC motor without relying upon physical rotary position detectors which add to motor size and cost.
It is known that, as a magnetic rotor rotates past stator windings, the rotor acts as a generator to induce signals in the windings, particularly those which are not then being energized. Currents flow through the windings in response to such induced back emf, and these currents together with the back emf appear as approximate sinusoidal waveforms. It has been proposed to use the back emf induced in a stator winding as a position signal analogous to the aforementioned Hall signal. See, for example, U.S. Pat. Nos. 3,997,823, 4,162,435, 4,262,236, 4,262,237, 4,446,406, 4,495,450, 4,651,069 and 4,712,050, as examples. However, the back emf induced in a winding is subject to electrical noise due, primarily, to so-called di/dt components in the windings. Furthermore, even if the back emf signal can be smoothed to provide generally a sinusoidal waveform, a limited number of position-representing signals (e.g., zero crossing pulses) can be derived from that back emf signal.
For example, in a 3-phase, 8-pole brushless DC motor, there are four electrical cycles in the back emf during one complete 360.degree. rotation of the motor. If the zero crossing points of the back emf component are detected for the purpose of generating position-representing pulses, one complete rotation of the motor provides only eight so-called position pulses. If each such position pulse is used to synchronize the energization of the stator phases, thereby "stepping" the rotor at each energization, less than desired motor control is achieved because only eight position pulses are available for each motor revolution.
It is believed that a far more accurate representation of position be obtained if the third harmonic component provides six zero crossing pulses for each full cycle of the fundamental. Hence, the third harmonic component would provide 24 position pulses for each complete revolution of the motor. It is, of course, necessary to extract the third harmonic component from the stator windings in order to exploit it.
One suggestion for detecting the third harmonic is found in the paper entitled "Electrical Drive Control of An Artificial Heart", by Marcel Jufer et al. In this paper, it is suggested that the third harmonic can be detected by sensing the difference between the center tap point of the stator windings and the center tap point of a parallel-connected resistance network. Since the voltages at the respective center taps are substantially equal, except for the harmonic component, the difference obtained therebetween is, essentially, constituted by the third harmonic component. The Jufer et al. paper does not suggest how this third harmonic component, or the zero crossing pulses derived therefrom, should be used to control the energization of the stator windings.
Another difficulty associated with DC brushless motors having no physical position detectors relates to the start-up operation of such motors. In a DC brushless motor having position detectors, the actual starting position of that motor is known from the signals produced by the position detectors, namely the Hall elements, just prior to rotation. Since the sequence in which the stator windings are energized determines the initial direction of rotation of the motor, it is important that the proper sequence be selected initially to avoid, for some applications, reverse rotation; and the position pulses assure that the proper sequence is initiated. However, in the absence of such position pulses, it is difficult, if not impossible, to know the actual starting position of the rotor at start-up. This is particularly true when it is recognized that the rotor probably rotated to some arbitrary position during shut-down of a previous motor operation. Since the energization of the proper phase or phases is dependent upon the actual position of the rotor, but that position is not known, there is a likelihood that the motor may commence reverse operation when a particular (or arbitrary) phase is energized during start-up.
To avoid this possibility of reverse motor movement and to select the proper phase and sequence for a start-up operation in accordance with the actual rotor position, it has been proposed to measure the inductance of the stator phases prior to rotor movement. It was thought that the initial position of the rotor affects the inductance of the respective phases and, therefore, if such inductance can be measured, actual motor position will be known.
Such inductance measurement may be attained by sequentially energizing the stator phases in the same sequence as used during normal motor operation, and observing the current rise times through each phase. Unfortunately, the inductance of the respective phases of one motor may differ significantly from the inductances of the same phases of another. Moreover, within a quarter revolution of the rotor, there are two different rotor positions at which the inductance measurements are substantially identical. Hence, if inductance is used as an indication of motor position, it still would not be known if the motor is at one or the other of these positions providing equal inductance.
When a particular phase (or phases) of the stator is supplied with a drive current while the motor is at rest, it is statistically possible for the rotor to move backward fifty percent of the time. When considering a 3-phase, 8-pole brushless DC motor, depending upon the actual position of the rotor and the phases to which drive currents are supplied, the rotor may move backwards by as much as 3/24 revolutions. (A 3-phase, 8-pole motor may be considered a stepper motor having 24 individual steps for one complete revolution. Such a motor typically is driven by supplying drive currents to six different combinations of phase windings. Statistically, currents supplied to three of these combinations will result in a forward rotation of the motor and currents supplied to the remaining three combinations will result in a reverse rotation at start-up. One of the combinations of phase windings will result in a reverse movement of three steps, thus providing 3/24 revolutions.) In moving backwards, the motor will accelerate until it reaches what otherwise would be an equilibrium point, but because of momentum, the rotating motor will overshoot this point by almost as much as the rotor had moved to reach it. Thus, reverse rotation of the motor may be as much as 3/24+3/24 revolutions, or up to 1/4 revolutions. For the environment in which the brushless DC motor is used as a spin motor for a rigid disk drive, a quarter revolution in the reverse direction may damage the disk drive heads.